1. Field of the Invention
The present invention relates to an image forming apparatus and a fixing apparatus provided thereto and, in more particularly, to an apparatus for forming an unfixed toner image on a surface of a recording material by appropriate image forming processing means such as electrophotography, electrostatic recording and magnetic recording including a copier, a printer and a facsimile, using a toner made from a heat melting resin and so on by a direct or indirect method, and heating and fixing it on the surface of the recording material as a permanently fixed image by heating and fixing means.
2. Related Background Art
In an image forming apparatus, a fixing apparatus of a heat-roller method is widely used as a heating means for fixing an unfixed toner image formed on a recording material by an appropriate image forming processing means. The fixing apparatus of the heat-roller method keeps in contact a fixing roller as a heating member incorporating a heat generating means such as a halogen heater and a pressure roller as a pressure member so as to fix the unfixed toner image by applying heat and pressure while transporting the recording material.
In recent years, a fixing apparatus of a film heating method is rendered commercially practical from viewpoints of a quick start and energy conservation. The fixing apparatus of the film heating method is the one wherein a fixing nip is formed by having a heat-resistant thin film sandwiched between a ceramic heater as heat generating means and a pressure roller as a pressure member. It fixes the unfixed toner image by rotating the film and the pressure roller together to apply the heat and pressure while transporting the recording material. The film is heated by the ceramic heater at the fixing nip. The ceramic heater has its temperature sensed by a temperature sensing element provided on the back thereof, and energization to the ceramic heater is controlled and temperature control thereof is performed based on the results of the sensing.
As for the above fixing apparatus of the film heating method, heat capacity of the film as a heating member is very small compared to the heat-roller method, and so it is possible to efficiently use thermal energy from the heat generating means in a fixing process. For this reason, a temperature rising speed of the fixing apparatus is fast so that waiting time between power-up of the apparatus and a printable state thereof can be rendered shorter (quick start). In addition, there is no need to preheat the heating member during standby for printing so that power consumption of the image forming apparatus can be held low (energy conservation).
There is a proposal, as a fixing apparatus of a further high-efficiency film heating method, of the fixing apparatus of the electromagnetic induction heating method for causing a conductive film itself to generate heat. Japanese Utility Model Application Laid-Open No. 51-109739 discloses, as the fixing apparatus of the electromagnetic induction heating method, the fixing apparatus for having an eddy current induced to a metallic film by an alternating magnetic field to cause the metallic film to generate heat with Joule heat. As it is possible to cause the film itself to generate heat by the electromagnetic induction heating method, the thermal energy from the heat generating means can be used further efficiently in the fixing process.
Hereafter, the temperature control of the fixing apparatus on a start of printing will be described.
FIG. 23 is a schematic view showing a fixing film temperature, a target temperature setting and timing of recording material reaching the fixing apparatus when starting the printing in the fixing apparatus of the past fixing apparatus (the fixing apparatus of the film heating method using the ceramic heater or the fixing apparatus of the electromagnetic induction heating method/film heating method).
Although the temperature control is off and no preheating is performed during standby for printing, preheating may also be performed. The image forming apparatus starts an image forming operation after receiving a print signal. The image forming apparatus starts power supply to the fixing apparatus at the same time, and increases the temperature of the fixing apparatus to a fixing temperature Tf. And the fixing apparatus keeps the fixing temperature Tf and prepares for fixing of the unfixed toner image on the recording material. The above steps will be collectively called a starting step of the fixing apparatus.
In the starting step of the fixing apparatus, the recording material are not put through paper so that most of the heat from the heat generating means is used to increase the temperature of the pressure roller via a film. In particular, in the case where the fixing apparatus is already warmed up, time twu for rising to the target temperature is short and time tp−twu for keeping the fixing temperature Tf is long, so that the temperature of the pressure roller further rises. For this reason, the temperature of the pressure roller is apt to rise excessively in the case where the starting step is repeated as with intermittent printing.
In the case of fixing the recording material requiring a lot of heat capacity for the fixing such as a cardboard or an OHT film in general, processing speed is reduced. In the stating step in such a case, time tp from the start of the image forming operation until the recording material reaches the fixing apparatus becomes longer, and so the time tp−twu for keeping the recording material at the fixing temperature Tf without putting it through paper becomes longer. For this reason, the temperature of the pressure roller is apt to rise excessively as with the intermittent printing.
As described above, there is a problem that, if the printing is performed in a state in which the temperature of the pressure roller has excessively risen, slipping of the recording material is apt to occur. It is because moisture in the recording material evaporates on the heating and fixing and frictional force between the pressure roller and the recording material is reduced. In particular, the higher the temperature of the pressure roller is, the more the amount of evaporated moisture becomes, and so the slipping of the recording material is more likely to occur. Furthermore, the slipping of the recording material occurs more conspicuously in the case of the fixing apparatus of the film heating method wherein a driving force is applied to the pressure roller and the film is rotated by being slaved to the pressure roller.
There was a problem that, if the slipping of the recording material occurs, the recording material does not move along a carriage guide member or winds itself around the film, resulting in occurrence of a jam. Furthermore, there was a problem that, as it is not possible to stably apply the heat and pressure to the unfixed toner image, quality of a fixed image is lowered.